I filed this too quickly: `chull()` fails generally if there are NAs in x and y at the same locations, not just at the first or last:
> set.seed(121222)
>
> nas <- sample(2:99, 20)
> x <- runif(100)
> y <- runif(100)
> x[nas] <- NA
> y[nas] <- NA
>
> length(chull(x,y))
[1] 16
> length(chull(x[-nas], y[-nas]))
[1] 10
Obviously returns of these two calls must contain different indices, but should
represent identical polygns and therefore have the same length...